在20世纪80年代至90年代初,网络构建的主要目的是通过计算机网络实现文件和打印机的共享。在这一阶段,文件和打印服务器(例如,NovellNetWare系统)是以一种非常消极的方式为客户端提供对共享资源的访问。用户必须登录到特定的网络服务器,才能访问共享目录。 这时的电子邮件系统多数采用共享文件的消息传递方式,其工作原理非常简单(如图1所示)。消息服务器拥有一个共享的文件夹(即邮箱),系统在这个集中的地点进行邮件的维护工作。当用户发送消息时,用户的邮件客户端把该消息复制到指定收件人的共享文件夹内,而客户端一般被用户配置成在 During the 1980s and early 1990s, the main purpose of network construction was to share files and printers over a computer network. At this stage, file and print servers (such as Novell NetWare systems) provide clients with access to shared resources in a very negative way. Users must log in to a specific web server to access the shared directory. At this time most of the e-mail system using shared file messaging, its working principle is very simple (Figure 1). Message server has a shared folder (ie, mailbox), the system in this centralized place for mail maintenance. When a user sends a message, the user’s mail client copies the message into the designated recipient’s shared folder, and the client is typically configured by the user to